Marco Rubio, who upended Florida politics and the ambitions of a popular governor, was sworn into office Wednesday and pledged to represent the voice of dissent that was so potent in November.

"I want to go to Washington, D.C., stand up to the direction it is taking our country and offer a clear alternative,'' he said. "That's what I ran on, that's what I'm going to be for the next six years."

The 39-year-old Republican, whose campaign platform was built on opposition to President Barack Obama's policies, said he would not be hemmed in by ideology, but so far finds little to agree on with Democrats.

"I think we should only oppose their bad ideas, and they have a lot," said Rubio, echoing his party's call to "repeal and replace" the new health care law and curb government spending.

Escorted by Democratic Sen. Bill Nelson and former Sen. Mel Martinez, whose resignation in 2009 created a drama-filled scramble to fill the seat, Rubio was sworn in at 12:28 p.m. by Vice President Joe Biden.

Clutching a Bible that was a gift from his 5-year-old son Anthony's kindergarten class, Rubio signed the oath of office and took his seat in the back row of the GOP side of the chamber.

For the first time in his political career, Rubio will be in the minority party. He is also at the bottom of the seniority list, though he is not the youngest senator; that is Utah Sen. Mike Lee, born a week after Rubio in 1971.

Rubio's moment was a touchstone in a career already steeped in them. At 35, he became the first Cuban-American speaker of the Florida House, garnering national attention for a failed plan to eliminate property taxes in exchange for a higher sales tax. That attention exploded during the Senate campaign after he seized voter discontent and GOP grass roots disappointment with the more moderate Gov. Charlie Crist.

Eventually Crist was forced to leave the GOP and run as an independent. Rubio drew strength from the tea party, but kept his distance and pressed familiar Republican themes.

It took time for Rubio to gain momentum, and he considered running for another office, perhaps state attorney general. Then he changed his mind, erased a double digit polling deficit and raised record amounts of money.

"It really personifies a lot of the Cuban immigrants in Miami,'' said former state Rep. Gaston Cantens, who was once in line to become the first Cuban-American House speaker. "They just persevered and overcame a lot of obstacles, and so did he."

In Washington's hyper news culture, Rubio has already been talked of as a vice presidential candidate in 2012, or even leading the ticket. He laughed off the hype Wednesday.

"It's a circus, you guys are part of the circus," he told reporters. "They'll talk about somebody else next week."

Speaking from his temporary, bare-bones office in the basement of the Dirksen Senate Office Building, Rubio said he felt humbled. "I don't feel I won anything other than the opportunity to serve."

There are signs of cooperation with Democrats. Rubio has praised the president's efforts at education reform and has talked with top Democrats, including former governor and senator Bob Graham.

Graham predicted Rubio will be pragmatic, without "a litmus test of ideology through which every decision is vacuumed."

Former U.S. Rep. Clay Shaw, a long serving Republican from South Florida, said Rubio faces the same problem as other Republicans carried by the November wave — expectations are high, particularly for tea party voters. "They are going to expect these guys to come in and make changes,'' he said.

"They are going to have to work together. Are they here to make legislation or make a statement?'' If they make their point with legislation, they can be "a very powerful force'' that is good for the nation, he said.

Rubio has talked with Nelson, and the two have pledged to work together on the issues facing Florida. Still, they already disagree about health care.

Such battles are for another day though. Rubio celebrated Wednesday with friends and family, including his mother, Oria, wife Jeanette and their children Amanda, Daniella, Anthony and Dominic. He held up his hands and said they were cracked — not from the bitter cold winds that have swept through the capital but from lacing up the winter boots his wife bought for the children.

When the school year is over, he and his wife will decide whether to move the family to the Washington area. He has not yet found a place to live and has been staying in a hotel. The family spent the past few days visiting Mount Vernon, the Museum of American History, monuments and the White House.

Rubio does not yet have committee assignments but Foreign Relations and Commerce, which oversees NASA, are relevant to Florida, as is Armed Services.

After the official swearing in, Rubio and his family participated in a ceremonial one in the Old Senate Chamber, where cameras are allowed. The new senator made his way through reporters and other senators and their families. At an elevator 8-year-old Daniella observed, "It says 'Senators only," as if she could not ride with her father.